The present invention relates to a control system through which two fluid power operated systems are supplied by a common pressure pump through separate branch conduits.
Fluid control systems of the foregoing type are generally known as disclosed in U.S. Pat. No. 4,174,018. In such systems, one branch conduit from the pump is connected to a flow divider from which a relatively low pilot flow volume of fluid is conducted to the steering control valve of a fluid power steering system while another residual flow conduit from the flow divider conducts operating fluid to another control valve for a power operated cylinder associated, for example, with a hydraulic lifting system. In the neutral position of the steering control valve, the pilot flow of fluid is returned to a reservoir tank and is therefore available for controlling the operating pressure of the fluid supplied to the vehicle steering system at the beginning of a steering maneuver. The other branch conduit from the pump bypasses the flow divider and is directly connected to the steering control valve. However, flow through the other branch conduit is blocked in the neutral position of the steering control valve which is so constructed that operating pressure for the steering system is regulated in response to adjustment of the steering control valve while the pilot flow fluid is conducted thereto. Branching of the output of the pump upstream of the flow divider thereby ensures that the fluid power cylinder of the steering system will receive that portion of the available flow necessary to meet the steering load demand.
In the foregoing type of interrelated steering and hydraulic working systems, the steering control valve must either be adjusted to a higher pressure in accordance with pressure conditions in the hydraulic working circuit or adjusted downwardly an initially higher pressure. The presence of a high pressure in the hydraulic working system is disadvantageous because such pressure must be reduced for maximum piston travel under control of the steering valve requiring a special valve surface configuration which differs from the optimum configuration otherwise dictated by the magnitude of the pressure in the hydraulic working system relative to the pressure in the steering system. Another disadvantage resides in the loading of the other structural components of the steering system by the possibly higher pressure of the hydraulic working system. Also, for the same steering resistance, the actuating force required on the steering wheel differs in dependence on the pressure level at which the hydraulic working system is operating at any instant.
It is therefore an important object of the present invention to provide a control system of the foregoing type which avoids loading of the steering system by the higher pressure of another hydraulic working system supplied with pressurized fluid from the same pump.